1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor thin film, an apparatus for manufacturing a semiconductor thin film, and a method of manufacturing a thin film transistor (TFT). More particularly, the present invention relates to a method of manufacturing a semiconductor thin film in which the formation of a grain boundary is controlled, and a thin film transistor having a layer of the semiconductor thin film.
2. Description of the Related Art
As a display of an electronic product such as a personal computer and the like, a liquid crystal display (LCD) is known and widely used. In particular, an active matrix liquid crystal display (AM-LCD) has been rapidly popularized in recent years, because high quality images can be achieved due to switching devices provided for respective pixels. In the AM-LCD, a thin film transistor (referred to as “TFT”, hereinafter) is used as a switching device for controlling a pixel and as a driver IC. In addition to a liquid crystal display, such a TFT is also used in a driving-circuit-integrated-type image sensor and a fluorescent display tube and the like.
FIGS. 1A to 1D show conventional processes to manufacture the above-mentioned TFT. At first, as shown in FIG. 1A, an amorphous silicon film 202 is formed on a surface of a glass substrate 100, for example. This amorphous silicon film 202 is a precursor film of a semiconductor layer which will be described later. Next, as shown in FIG. 1B, a laser light 300 is irradiated to the surface of the amorphous silicon film 202, and crystal grains are grown. As a result, a polycrystalline silicon (poly-silicon) film 102 is formed from the amorphous silicon film 202. Here, the laser annealing is carried out by scanning the laser light 300 from one end (for example, the left end in FIG. 1B) of the amorphous silicon film 202 to the other end (the right end in FIG. 1B).
Then, as shown in FIG. 1C, a gate insulating film 104 is formed on the formed poly-silicon film 102. Then, the channel-doping is performed on a channel region 120 of the semiconductor layer (poly-silicon film) 102. After that, a gate electrode 106 is formed on the gate insulating film 104.
Next, as shown in FIG. 1D, a first inter-layer insulating film 130 is formed so as to cover the gate electrode 106 and the gate insulating film 104. Then, contact holes 108a, 110a are formed to penetrate the first inter-layer insulating film 130 and the gate insulating film 104. Then, a source electrode 108 connected to the contact hole 108a and a drain electrode 110 connected to the contact hole 110a are formed on the first inter-layer insulating film 130. Then, a second inter-layer insulating film 132 is formed so as to cover the source electrode 108 and the drain electrode 110. Thus, the TFT is manufactured.
In recent years, the fineness of an LCD tends to be increasingly improved, and also the higher performance is required for an LCD to support the higher resolution moving image. Therefore, a TFT used to control the pixels is desired to operate faster. A TFT can operate faster as the mobility of carrier (electron or hole) in the poly-silicon film 102 increases. However, if there are a large number of grain boundaries in the poly-silicon film 102, the mobility of the carrier decreases, which results in a problem that the mobility of the TFT can not be made faster.
Therefore, a technique has been proposed, in which the crystal growth during the laser annealing process is controlled in order to reduce the number of the grain boundaries in the poly-silicon film 102 and hence to improve the mobility of the carrier.
Japanese Laid Open Patent Application (JP-P-2002-217206) discloses a technique in which a rectangular laser line beam is irradiated to form a poly-silicon layer which has crystal grains extending for a length equal to about half of a width of the laser line beam.
FIG. 2 is a schematic picture for explaining this technique. According to this technique, the rectangular laser line beam with the size of about 5 μm×100 μm is irradiated to an amorphous silicon layer 303 shown in FIG. 2. At this time, the profile of the laser beam energy density is trapezoidal as shown on the left side of FIG. 2. As a result, silicon seed crystals (not shown) are randomly generated in portions of the amorphous silicon layer indicated by Y1 and Y2 which correspond to the ends of the laser line beam. Then, poly-silicon grows from those silicon seed crystals toward a portion which corresponds to the center of the laser line beam and is indicated by Y3. The growth of the poly-silicon stops at the portion Y3. In this way, a poly-silicon layer 303′ (331, 332) is formed, which has crystal grains extending for the length equal to about half of the width of the laser line beam. Here, the grain boundaries extend from the portion Y1, Y2 to the portion Y3.
According to this technique, it is possible to make the mobility of the thin film transistor and ON-state current higher by setting the growth direction of the crystal (poly-silicon) into alignment with a carrier running direction in the thin film transistor. However, there is a problem with this technique in that the formation of the grain boundaries can not be controlled, even though the growth direction of the crystal can be controlled to some degree. If the grain boundary crosses the channel portion of the thin film transistor, the desired mobility of the carrier can not be obtained. Also, the diameter of the crystal grain is at most the length equal to the half of the width of the laser line beam. Thus, there is also a problem with this technique in that poly-silicon with a large grain diameter can not be formed.
As another technique, James S. Im et al. reports a technique in which a narrow beam is scanned to form a giant crystal grain in the scanning direction (refer to a non-patent document: “Sequential lateral solidification of thin silicon films on SiO2”, Robert S. Sposili and James S. Im, Appl. Phys. Lett 69 (19) 1996 pp. 2864-2866).
FIGS. 3A to 3E schematically show the processes according to this technique. In this technique, the narrow beam 310 shown in FIG. 3A is made from a pulse laser light by using an appropriate mask. The narrow beam 310 has a width of 5 μm and a length of 200 μm, and is irradiated to an amorphous silicon film 202. This narrow beam 310 is to be scanned along the direction E shown in FIG. 3A, i.e., from one side (indicated by a numeral D in FIG. 3A) to the opposite side (indicated by a numeral S in FIG. 3A). Thus, the amorphous silicon film 202 is to be sequentially heated (annealed) as shown in FIGS. 3B to 3E.
FIG. 3B shows a situation when the first irradiation of the narrow beam 310 is finished. At this time, the crystallization (solidification) of the melted amorphous silicon film 202 starts from boundaries between “the melted region” and “the non-melted region” (indicated by two-dot chain lines in FIG. 3B). Here, these boundaries correspond to the ends of the narrow beam 310 (top and bottom ends in FIG. 3B). Then, the growth of the crystals begins from those boundaries toward the center of the melted region. In this way, the solidified portion becomes a crystallized poly-silicon film 102. The growth of the crystals stops when they collide with each other near the center. Thus, a grain boundary B is formed near the center portion as shown in FIG. 3B. It should be noted that the crystallization advances in the lateral direction in FIG. 3B, and a lot of grain boundaries are formed along the longitudinal direction. These longitudinal grain boundaries will be described later.
Next, as shown in FIG. 3C, the irradiation region (the narrow beam 310) is moved upward, and the second irradiation of the narrow beam 310 is performed. Here, the displacement of the narrow beam 310 is 0.75 μm.
Similarly, the crystallization (solidification) starts from boundaries between “the melted region” and “the non-melted region” as shown in FIG. 3D. These boundaries are indicated by two-dot chain lines in FIG. 3D, and correspond to the ends of the narrow beam 310. Crystals grow from those boundaries toward the center of the melted region, and a grain boundary B′ is formed near the center portion as shown in FIG. 3D. At this time, the grain boundary B formed in the previous step disappears due to the melt. Also, the melted portion along the lower boundary crystallizes based on the crystal formed by the previous step (the first irradiation). Therefore, no lateral grain boundary is formed along this lower boundary.
After that, the irradiation region (the narrow beam 310) is moved and the irradiation of the narrow beam 310 is performed sequentially. The melt and crystallization of the amorphous silicon film 202 are repeated in the similar way. Thus, the crystal grains extending in the longitudinal direction (the scanning direction E) can be formed as shown in FIG. 3E, and no grain boundary is formed along the lateral direction (orthogonal to the scanning direction E).
Also, Matsumura et al. reports a technique that uses a light-shield plate in laser annealing (refer to a non-patent document: “Excimer-Laser-Induced Lateral-Growth of Silicon Thin Films”, Kensuke Ishikawa, Motohiro Ozawa, Chang-Ho Oh and Masakiyo Matsumra, Jpn. J. Appl. Phys. Vol. 37 (1998) pp. 731-736). FIG. 4A is a schematic picture for explaining this technique, and FIG. 4B is a cross sectional view along a dashed line J-J′ in FIG. 4A.
According to this technique, a light-shield plate 400 is placed in a light path to shade a part of the narrow beam 300A, as shown in FIGS. 4A and 4B. A part of the narrow beam 300A is diffracted to be a diffracted beam 310A as shown in FIG. 4B, and the energy density of this diffracted beam 310A becomes low. Therefore, the temperature of the amorphous silicon film 202 associated with the diffracted beam 310A becomes lower than that associated with the directly incoming narrow beam 300A. In other words, the temperature of the irradiation region on the left side of FIG. 4B is higher than the temperature on the right side, i.e., a temperature gradient is generated as indicated by an arrow in FIG. 4B. This temperature gradient promotes the growth of the poly-silicon film 102.
According to the conventional techniques mentioned above, the growth direction of the crystal can be controlled to some degree. However, in the technique disclosed in the above-mentioned patent document, the formation of the grain boundaries can not be controlled. There is also a problem with the other conventional techniques in that the grain boundaries along the scanning direction can not be controlled.
FIG. 5 is a schematic picture explaining such a problem in the conventional technique. In FIG. 5, the longitudinal direction I denotes the scanning direction. As shown in FIG. 5, crystals grow along the direction I. However, each crystal can not grow largely in the direction orthogonal to the direction I, and a lot of grain boundaries B along the direction I are generated in the poly-silicon film 102.
Here, the formation of these grain boundaries B is not controlled, i.e., the locations and the distribution density of the grain boundaries B are not controlled. Therefore, there is a certain distribution in the distances between grain boundaries B adjacent to each other (each distance corresponds to a width t of each crystal 102a). The maximum value of the distance is about 1 μm. Even if the movement direction of the carriers in the TFT is set to the direction I, the movement of the carriers is interfered by the grain boundaries B which are bent in the lateral direction (orthogonal to the direction I) or intersect with other grain boundaries B. This causes the depression of the carrier mobility. Moreover, the widths t of the respective crystals 102a are not constant and vary depending on the manufacturing condition. Furthermore, the number and the directions of the grain boundaries B also vary. Thus, the carrier mobility and the threshold voltage of the manufactured TFT vary depending on the manufacturing condition.
The following may be considered as a reason for the formation of the crystals 102a with the various widths t. When the first irradiation of the laser light 300 is performed on the amorphous silicon film 202, the temperature gradient is not generated along the direction orthogonal to the direction I in the melted amorphous silicon film 202. Therefore, crystal seeds are formed randomly in the direction orthogonal to the direction I. Then, the crystals grow based on the crystal seeds and extend along the direction I as mentioned above. As a result, various crystal grains with various widths t are formed as shown in FIG. 5.